From the viewpoint of an energy issue or an environment issue in recent years, various investigations are made in order to utilize clean natural energy that can replace fossil energy sources such as petroleum. For example, hydro-electric power generation, in which drop energy of dammed water is used, is implemented as electricity generation of the natural energy. However, in order to perform the hydro-electric power generation, it is necessary to construct a dam that retains a water-level difference, a large amount of construction cost is required, and an environmental load also increases. On the other hand, nowadays, attention is paid to existence of a place where a large water flow (the tidal current, the ocean current, and the river current) is generated in the sea and river, and a floating plant that generates electricity using water flow energy has been proposed (for example, see Japanese Unexamined Patent Publication No. 2009-8098).